Heretofore, in various devices, for example, as a method of transmitting information signal from a sensor to an electronic control unit (hereinafter referred to as ECU), an analogue communication system and a digital communication system have been in popular use. For example, in the analogue communication system, to detect communication anomaly of signal lines and the like, it is possible to set a detection threshold value for an abnormal voltage by using an analogue voltage as an information signal. On the other hand, in the digital communication system, it is possible to detect the communication anomaly by checking a parity data included in the information signal (digital signal).
In the detection of the communication anomaly by the digital communication system described above, at the time of communication anomaly, an anomaly region (failed region) or a cause of the anomaly cannot be specified.
Further, there is known a communication device, in which a sensor and an ECU are connected through communication lines in such a manner that bidirectional digital communications are possible (for example, Patent Document 1). Adopting such a communication system, for example, to an occupant detector for determining an occupant sitting in a seat of a vehicle such as an automobile has been proposed.
That is, the occupant detector comprises a plurality of load sensors for acquiring load information according to a load applied to a seat and the ECU connected to these load sensors through each signal line in such a manner that bidirectional digital communications are possible. When the ECU transmits an information request signal to a plurality of load sensors, respectively, these load sensors receive the information request signals. The load sensors transmit the load information signal including the load information in response to the received information request signal. The ECU receives the load information signals and performs an occupant determination.
The ECU comprising such an occupant detector transmits the information request signals in order to these load sensors from the same number of plural transmission ports as the plural load sensors. Consequently, it is often the case that a time lag is caused to the timing in which the ECU transmits the information request signal to each load sensor. Hence, synchronicity of the load information included in the load information signals transmitted from these load sensors is often impaired. That is, each load sensor transmits the load information signal including the load information acquired by mutually different timing. On the other hand, a posture of the occupant sitting in a seat is constantly changing, and the load information acquired by these load sensors is also constantly changing. Consequently, in a case where the ECU receives the load information signals including these pieces of the load information impaired in synchronicity and performs the occupant determination based on such received load information signals, accuracy of the occupant detection is often lowered.
Further, heretofore, with respect to disconnection detection adoptable to the occupant detector for performing an occupant determination by detecting a load applied to the seat of the vehicle such as an automobile and the like, there is known, for example, the detector disclosed in Patent Document 2. As shown in FIG. 9, in a normal operating status of this detector, an output transistor Q increases and decrease the current flowing between a collector and an emitter according to the detection voltage of a sensor 210 applied to a base. Accompanying this, a voltage drop generated at a resistor R201 and a resistor R202 changes, and a voltage EOUT of a signal output terminal 213 increases and decreases. Hence, the voltage drop generated at a resistor R204 of a converter 220 connected to the signal output terminal 213 changes. Based on this voltage drop generated at the resistor R204 according to the detection voltage of the sensor 210, the status of a monitoring object is detected.
In such constitution, when a lead wire (wire harness 230) between a power supply terminal 212 of the sensor 210 and a power supply terminal 222 of the converter 220 is disconnected (opened), the emitter of the output transistor Q is opened from a ground GND and is put into an OFF state. For example, by making the voltage drop generated in the resistor R204 higher than the maximum value of the voltage generatable in a normal operating state, the securing of a clamp voltage (for example, 4.4V or more) of a H level in an OFF state of the output transistor in order to detect disconnection is considered. For this purpose, the voltage of the signal output terminal 213 of the sensor 210 is required to be set high in advance.
The voltage of the signal output terminal 213 in an off state of the output transistor Q is determined by a combined resistance of resistors R203 and R204 and a partial pressure ratio with the resistor R201. Consequently, opening (disconnection) between the power supply terminals 212 and 222 is detectable by setting the resistor R204 sufficiently larger (for example, 100 kΩ or more) than the resistor R201.
In a case where such a constitution is adopted, for example, in a case where the power supply voltage E is 5V, the output current from the signal output terminal 213 to a signal input terminal 223 becomes small, which is below 0.05 mA (≈5V/100 kΩ). In a case where the signal output terminal 213 and signal input terminal 223 are general purpose terminals tin-plated with copper, the current flow becomes little, and therefore, it becomes difficult for the current to crush an oxide film formed in the signal output terminal 213 and the signal input terminal 223. Alternatively, there arises a need to treat the signal output terminal 213 and the signal input terminal 223 with a gold plating as a counter measure against the oxide film, so that there is no choice but to increase the number of manufacturing man-hours and manufacturing cost.
Further, since the resistance value of the resistor R204 is high, a signal system line (between the signal output terminal 213 and the signal input terminal 223) becomes high in impedance, and therefore, being easily affected by peripheral noises and the like, the output current from the signal output terminal 213 of the sensor 210 to the signal input terminal 223 of the converter 220 often changes.
Patent Document 1: Japanese Patent Laid-Open No. 2002-188855
Patent Document 2: Japanese Patent Application Laid-Open No. 5-107292